This invention relates to a nozzle cap and a method for sealing a nozzle. More particularly, this invention relates to a nuclear power reactor in which the nozzle cap is used to seal penetrations into the nuclear reactor vessel.
In many industrial applications, particularly in nuclear power related technologies, penetrations must be made in large vessels within which processes occur at high pressure. In the case of a nuclear reactor vessel, such penetrations are necessary for providing openings through which control rods or instrumentation may be inserted into the reactor core. Typically, nozzles are welded to the vessel wall to provide support surfaces on which other equipment associated with the manipulation of the control rods or instruments, may be mounted.
A pressurized water reactor (PWR) vessel used in a nuclear facility includes a nuclear control rod drive mechanism mounted on such a nozzle in the reactor vessel head to control movement of an associated neutron absorbing control rod. A nuclear reactor vessel, showing a control rod drive mechanism housing connected to a reactor vessel nozzle extending from the reactor vessel head, is depicted in FIG. 1. A nuclear reactor system 10 is shown having a reactor 12 including a substantially upright cylindrical vessel 14 and a substantially semi-hemispherical head 16. The vessel has an upper flange 18 and the head has a lower flange 20 which are secured together for normal operation of the reactor system 10. The nuclear power generated in the reactor can be controlled by an increase or decrease in the amount of neutron absorbing material that is inserted into or withdrawn from the core in the vessel 14. This is typically accomplished by a plurality, normally at least several, control rod clusters or assemblies (not shown) each of which pass through a nozzle 22. Only one of the reactor vessel nozzles 22 in the head is shown, but it should be appreciated that normally there are many nozzles and control rod assemblies that are moved into and out of the vessel 14 during operation. Each control rod drive assembly includes a control rod drive mechanism housing 24, which has an associated motor (not shown) for effectuating the movement of the control rods.
As shown in U.S. Pat. No. 5,631,936 which is assigned to the assignee of this invention ('936 Patent), the housing has an omega lip including a free end, and the reactor vessel nozzle has a complementary omega lip which includes a semi-rigid, annular seat member placed within one of the lips. The housing is positioned in vertical alignment with the nozzle so that the free end of the housing lip and the free end of the nozzle lip are juxtaposed, thereby forming an omega structure having a cavity occupied by a seat member, a gap between the free ends of the lips, and an outer surface on either side of the gap. A semi-rigid ring, or omega seal, is positioned in conforming relation to the outside surface of the omega structure. Thus, a compressive force applied and maintained on the exterior of the seal ring extrudes a portion of the ring into the gap.
A number of other features of the relationship between nozzles and a reactor head are also discussed in the '936 patent which is hereby incorporated by reference for purposes of completeness of this disclosure.
Another example of a boundary seal for vessel penetration is shown in U.S. Pat. No. 4,655,483 ('483 Patent), which is assigned to the assignee of this invention. The disclosure of the '483 patent is also hereby incorporated by reference for purposes of completeness of this disclosure.
Occasionally, the equipment mounted on the nozzle must be removed for maintenance or repair. When maintenance on the control rod drive mechanisms is required, or when the control rod drive mechanisms are replaced, omega seal welds joining the housing components are cut, the housings removed, and new assemblies installed. These procedures involve laborious, time-consuming processes as discussed, for example, in the '936 Patent. Furthermore, prior to replacement of control rod drive mechanisms, monitoring of omega seals can be time consuming, and the omega seals may deteriorate with time.
Accordingly, it is generally desired to remove control rods or instrumentation that may be inserted into the reactor core, and prior to their replacement, to cap one, some, or all of the nozzles on the reactor head. It has been proposed that a suitable cap should include a rigid element to be welded to a reactor head nozzle. However, a potential problem exists with such a cap in that a rigid element would be subjected to undue stresses because of relative movements between the nozzle and the cap.
It is therefore an object of the present invention to improve upon the seals for nozzles in a rector head vessel compared to the seals in the prior art.
It is a further object of the present invention to provide a nozzle cap suitable for capping or closing any nozzle in a nuclear reactor head.